1. Field of the Invention
This invention relates to a phase-type diffraction device to be used as a low pass filter in an image pick-up device and the like wherein a large number of light-receptive pixels each formed, for example, of a charge-coupled device (CCD), a complementary metal oxide-semiconductor (CMOS) are periodically arranged, and also relates to the manufacturing method of the phase-type diffraction device. This invention also relates to an image pick-up apparatus employing the diffraction device.
2. Description of the Related Art
In an image pick-up optical system employing an image pick-up device constituted by a CCD, CMOS and the like, a colored light component (moire) which is different from the light originating from a subject body is caused to generate concomitant with the generation of false signals. In order to suppress the generation of moire, an optical low pass filter which restricts a high spatial frequency of the light of the subject body is generally employed.
With respect to the optical low pass filter, a birefringence type filter which utilizes the separation of ordinary ray and extraordinary ray in a birefringent substance is conventionally widely used. The materials constituting the birefringence type optical low pass filter which have been most widely employed are formed of inorganic crystals such as quartz, lithium niobate, etc. The birefringence type optical low pass filter made from an inorganic crystal is formed into a plate-like body through processes wherein a single crystal is synthesized at first and then subjected to post workings such as cutting, polishing, etc. Because of these processes, the conventional optical low pass filter is accompanied with the drawbacks that considerable time and labor are required for the manufacture thereof.
With a view to overcome these problems, there has been proposed the employment of a photo-reactive liquid crystal composition as a material for the birefringence type optical low pass filter. For example, Jpn. Pat. No. 3592383 describes a method wherein a thin film of a photo-reactive liquid crystal composition is formed at first and then polymerized in such a manner that the mesogen of liquid crystal is obliquely inclined from the normal direction of the thin film, thereby making it possible to obtain desired optical properties. Alternatively, JP-A 2007-93918 describes a method wherein a thin film of a photo-reactive liquid crystal composition is polymerized in such a manner that the inclined angle of the mesogens from the normal direction is continuously changed in the thickness-wide direction of the thin film, thereby obtaining desired optical properties.
However, it is difficult to uniformly orientate the mesogens in an inclined angle from the normal direction, especially at an angle of 20 to 70 degrees. When it is tried to create a structure wherein the inclined angle of the mesogens from the normal direction is continuously changed in the thickness direction of the thin film, since the separating width of the beam becomes smaller relative to the thickness of film, the thickness of the thin film is required to be increased.
Meanwhile, with respect to the optical low pass filter, a diffraction grating type device which utilizes the separation of light by diffraction is also widely employed other than the system which separates the beam of light using birefringence. With respect to the diffraction grating type low pass filter, one which is most commonly employed is a phase-type diffraction device. As described in JP-A 4-9803 for example, this phase-type diffraction device is constructed such that it comprises a plate having a periodical recessed/projected pattern on its surface created by a transparent material such as a transparent resin, thereby making it possible to create a phase difference of light between the recessed portions and the projected portions.
The phase-type diffraction grating utilizing the recessed/projected pattern is accompanied with a problem that a shadow of the grating is more likely to be generated due to the non-flatness of the surface of the device. Further, this phase-type diffraction grating is accompanied with a problem that it is difficult to enhance the diffraction efficiency, so that this phase-type diffraction grating is not suited for use in an image pick-up apparatus which is intended to obtain images of high-quality.